Braking systems, including parking brake systems, for vehicles have become increasingly complex and costly to manufacture over recent years. As braking systems in general have become more complicated, so have the mechanisms for their control. The increased complexity of the brake control mechanisms have typically resulted in the shift away from the pneumatic or hydraulic-based control mechanisms to an electronic-based control mechanism.
A significant problem with an electronic-based control mechanism for a braking system is the consequences posed by an unexpected loss of electrical power. Such electrical power failure may create a dangerous condition leading to bodily injuries and property damage. The prior art discloses brake control systems designed to prevent such a hazardous condition from occurring in the event of power failure by providing an additional mechanically actuated means for controlling brake application.
For example, U.S. Pat. No. 5,443,306 ("the '306 patent") to Broome discloses an electronically controlled braking system for a motor vehicle which includes a foot-operable brake valve 11 for electronically controlling brake application (col. 4, lines 48-56). The foot valve 11 includes a body 60 defining an exhaust port 78 which houses solenoid valves having valve members 98, 99, an abutment member 68, a coil spring 66, a tapered coil compression spring 76, a modulating piston 62 having an annular downward extension 75, a valve member 80 having a valve seat 81, a compression spring 82, and a differential piston 91 (col. 9, line 58-col. 10, line 16; FIG. 2). When there is failure of the electronic system, the solenoid valves of the foot valve 11 are de-energized and the above-described components of the foot valve 11 interact to allow the foot valve 11 to function as a standard pneumatic foot brake operating valve (col. 9, line 58-col. 10, line 22).
The disadvantages posed by the '306 patent are complexity and cost. Given that the foot valve 11 includes numerous discrete components which have to be assembled together (see FIG. 2), some of which have been described above, the electronically controlled braking system of the '306 patent is complex to manufacture. Furthermore, this complexity and the number of components needed make the electronically controlled braking system of the '306 patent costly to manufacture.
U.S. Pat. No. 5,370,449 ("the '449 patent") to Edelen discloses an electrically controlled parking brake system which includes an electrically operated valve 20 and a mechanical valve 18 (col. 2, lines 18-36). The mechanical valve 18 is mechanically actuated by a valve actuator 22, and the electrically operated valve 20 is actuated by a pair of solenoids 34, 36, which are controlled from a transmission shift console 40 (col. 2, lines 18-55; FIG. 1). The electrically operated valve 20 is held in position following actuation of the solenoids 34, 36 by a mechanical detent 38, so that in the event of power loss, the valve 20 will retain the previous position (col. 2, lines 36-52). If the last position of the valve 20 retained was the parking brakes applied position, then in the event of power loss, the parking brakes will remain applied (col. 2, lines 39-47). If the last position of the valve 20 retained was the parking brakes released position, then in the event of power loss, the control of the brakes is returned to the mechanical valve 18 (col. 2, lines 47-52).
Like the disadvantages presented by the '306 patent, the disadvantages posed by the '449 patent are complexity and cost. It is complex and costly to manufacture the electrically controlled parking brake system of the '449 patent since there are required the mechanical valve 18, the electrically operated valve 20 with solenoids 34, 36, and numerous components required to interconnect the two valves.
What is desired, therefore, is a parking brake control assembly which provides redundant control outputs to ensure parking brake integrity in case of electrical, hydraulic or pneumatic failure, and which provides cost and ease of manufacturing advantages over the prior art systems.